Magnetic Resonance Imaging of the Functional Anatomy of the Inferior Rectus Muscle in Superior Oblique Muscle Palsy

Purpose: Biomechanical modeling consistently indicates that superior oblique (SO) muscle weakness alone is insufficient to explain the large hypertropia often observed in SO muscle palsy. Magnetic resonance imaging (MRI) was used to investigate if any size or contractility changes in the inferior rectus (IR) muscle may contribute. Design: Prospective, case-control study. Participants: Seventeen patients with unilateral SO muscle palsy and 18 orthotropic subjects. Methods: Surface coils were used to obtain sets of contiguous, 2-mm-thick, high-resolution, coronal MRI views in different gazes. Cross-sectional areas of the IR and SO muscles were determined in supraduction and infraduction for evaluation of size and contractility. Diagnosis of SO muscle palsy was based on clinical presentations, subnormal contractility, and SO muscle size less than the normal 95% confidence limit. Main Outcome Measures: Cross-sectional areas of the IR and SO muscles. Results: Patients had 15.9 +/- 7.2 prism diopters (Delta; mean standard deviation) of central gaze hypertropia and exhibited ipsilesional SO muscle atrophy and subnormal contractility. Mean ipsilesional, contralesional, and normal IR muscle cross-sections were 28.5 +/- 3.5 mm(2), 31.9 +/- 3.8 mm(2), and 31.8 +/- 5.8 mm(2), whereas mean contractility was 16.5 +/- 3.8 mm(2) 20.5 +/- 4.1 mm(2), and 16.6 +/- 4.8 mm(2), respectively. Ipsilesional IR muscle cross-section and contractility was significantly less than contralesional cross-section and contractility (P < 0.01). Conclusions: In SO muscle palsy, the contralesional IR muscle is larger and more contractile than the ipsilesional IR muscle, reflecting likely neurally mediated changes that augment the relatively small hypertropia resulting from SO muscle weakness alone. Recession of the hyperfunctioning contralesional IR muscle recession in SO muscle palsy is a physiologic therapy.